Optical disk system having an objective lens with a numerical aperture related to the thickness of the protective layer

ABSTRACT

A magneto-optical disc system includes a magneto-optical disc 40 with a light-transmitting layer 41, an objective lens 2 for bundling or focusing a laser beam onto a magnetic recording layer 43 of the magneto-optical disc 40 for recording information thereon and/or reproducing information therefrom, and a magnetic field generating unit 9 having a coil pattern 7 arranged on an optical glass element 8. The thickness t2 of the light-transmitting layer 41 falls within the range of 0.6 to 0.1 mm, and the numerical aperture (NA) of the objective lens 2 falls within the range of 0.55 to 0.70.

This is a Continuation of application Ser. No. 08/613,035 filed Mar. 8,1996 and now is U.S. Pat. No. 5,757,733, which is a Continuation ofapplication Ser. No. 08/383,351 filed Feb. 3, 1995 and now is U.S. Pat.No. 5,533,009, which is a Divisional of application Ser. No. 08/277,357filed Jul. 19, 1994, now U.S. Pat. No. 5,392,263, which is aContinuation of application Ser. No. 07/761,874, filed Sep. 13, 1991,now abandoned.

TECHNICAL FIELD

The present invention relates to optical and magneto-optical discsystems for performing recording and/or reproduction of an informationsignal, and optical and magneto-optical disc used in these systems.

BACKGROUND ART

A magneto-optical disc system utilizing a magneto-optical effect isshown in FIG. 1 as an optical disc system for performing recording inand/or reproduction from an optical recording medium such as an opticaldisc or a magneto-optical disc.

A conventional magneto-optical disc system shown in FIG. 1 is arrangedsuch that when a disc-like one-sided magneto-optical recording medium 50is mounted, an optical system including a laser unit 58, an objectivelens 59, and the like is arranged above the one-sided magneto-opticalrecording medium 50, and a magnetic field generation unit 60 serving asa magnetic system is arranged below the one-sided magneto-opticalrecording medium 50. A numerical aperture (to be referred to as an NAhereinafter) of the objective lens 59 is set to fall within the range of0.50 to 0.53.

In order to drive the optical system in focusing and tracking directionswith respect to the magneto-optical recording medium 50, a drive system(not shown) is arranged. Another drive system (not shown) is arranged todrive the magnetic system in a direction indicated by an arrow in FIG. 1and the tracking direction.

In this magneto-optical disc system, a magnetic field modulation schemeis employed to perform recording. In this magnetic field modulationscheme, high-speed reverse control of a magnetic field must be performedin accordance with an information signal to be recorded. For thisreason, a sufficiently large excitation current cannot be obtained. Theintensity of the magnetic field generated by the magnetic fieldgeneration unit is limited. The magnetic field generation unit 60 islocated near a magnetic recording layer 53 (to be described later) inthe one-sided magneto-optical recording medium 50. An overwriteoperation can be performed according to the magnetic field modulationscheme.

The one-sided magneto-optical recording medium 50 is formed as follows.A dielectric layer 52, a magnetic recording layer 53 having a largemangeto-optical effect and made of, e.g., a rare earthelement-transition metal alloy amorphous thin film, a dielectric film54, a reflecting layer 55, a protective cover 56 are sequentiallystacked on one surface of a light-transmitting transparent substrate 51made of, e.g., polycarbonate. The transparent substrate 51 has apredetermined thickness t₁. The thickness of a conventional substrate isset to be 1.2 mm.

An operation of the magneto-optical disc system will be described below.

The one-sided magneto-optical recording medium 50 is placed and drivenon a rotary disc (not shown), and a magnetic field is applied from themagnetic field generation unit 60 to the magnetic recording layer 53 ofthe one-sided magneto-optical recording medium 50. High-speed reversecontrol of the magnetic field to be applied is performed on the basis ofan information signal to be recorded. A laser beam emitted from thelaser unit 58 is focused through the objective lens 59 on the magneticrecording layer 53 applied with this magnetic field. A change indirection of magnetization occurs in a region of the magnetic recordinglayer 53 irradiated with the focused laser beam in accordance with thedirection of the magnetic field applied from the magnetic fieldgeneration unit 60. Therefore, an overwrite operation of an informationsignal can be performed in real time.

In order to obtain a more compact magneto-optical pickup, which isconstituted by the optical system, the magnetic system, and the drivesystems for driving the optical and magnetic systems, it is assumed thatthe optical system may be formed integrally with the magnetic system andboth the systems are arranged on the one side of the magneto-opticalrecording medium. More specifically, the magnetic field generation unit60 of FIG. 1 is located on the side where the objective lens 59 islocated (i.e., the side of the transparent substrate 51). However, sincea distance between the magnetic field generation unit 60 and themagnetic recording layer 53 becomes larger than before, a sufficientlyhigh magnetic field cannot be applied to the magnetic recording layer53.

Along with an increase in information volume in recent years, atwo-sided magneto-optical recording medium has been developed, in whichthe magnetic recording layers are formed respectively on both surfacesof one magneto-optical recording medium so as to be capable of recordinginformation signals on each of the surfaces.

In order to perform recording in and/or reproduction from such two-sidedmagneto-optical recording media, it is very difficult to apply asufficiently high magnetic field to each of the magnetic recordinglayers by means of the magneto-optical pickup of FIG. 1, constituted bythe optical and magnetic systems. The reason for this is such that inthe magnetic field generation unit of a magnetic field modulationscheme, a high-frequency current corresponding to a high-frequency datasignal that is an information signal to be recorded must be supplied toan electromagnetic coil. A current tends to become difficult to flowthrough an electromagnetic coil as the frequency of the currentincreases, so that the generated magnetic field is limited. In addition,another reason is that the distance between the magnetic fieldgeneration unit and each magnetic recording layer is rather large.Therefore, in state-of-the-art techniques, it is very difficult toperform two-side optomagnetic recording under the magnetic fieldmodulation scheme.

In order to cope with an increase in information volume, moreinformation signals must be recorded in, e.g., a recording portion suchas the magnetic recording layer of an optical disc.

DISCLOSURE OF INVENTION

It is, therefore, an object of the present invention to provide alarge-capacity optical disc system capable of performing recordingand/or reproduction in higher density, and an optical disc used in thisoptical disc system.

According to an aspect of the present invention, there is provided anoptical disc system for recording an information signal in an opticaldisc and/or reproducing the information signal from the optical disc,the optical disc being provided with a recording layer irradiated with alaser beam to perform recording or reproduction and a light-transmittingcover which is formed to cover the recording layer and through which thelaser beam is transmitted, and the optical disc system comprising laserbeam generating means for generating the laser beam and an objectivelens for bundling or focusing the laser beam on the recording layerthrough the light-transmitting cover, characterized in that a thicknessof the light-transmitting cover of the optical disc falls within a rangeof 0.6 to 0.1 mm, and a numerical aperture of the objective lens fallswithin a range of 0.55 to 0.70.

When the laser beam is bundled or focused by the objective lens, theconvergent rays have a minimum diameter (2ω₀) represented by equation(1) below:

    2ω.sub.0 =0.82×λ/NA                     (1)

(λ: wavelength of laser beam)

Since the numerical aperture NA of the objective lens is larger than avalue falling within the conventional NA range of 0.50 to 0.53, theminimum diameter of the convergent rays is decreased, so that therecording density is increased. Therefore, reproduction corresponding tothis high recording density can be performed.

As the NA is increased, the thickness of the objective lens isincreased, but the thickness of the light-transmitting cover of theoptical disc is smaller than the conventional thickness (1.2 mm), sothat the objective lens having a larger thickness will not be broughtinto contact with the optical disc.

As the NA of the objective lens and the thickness t of thelight-transmitting cover of the optical disc are changed, the aberrationvalues of the objective lens are changed as follows.

(A) Spherical Aberration W₄₀ ##EQU1## (sinα=NA) (N: refractive index oflight-transmitting cover of the optical disc)

(B) Coma W₃₁ ##EQU2## (θ: skew)

The spherical aberration (A) can be corrected by the objective lens anddoes not pose any decisive problem. If the thickness t of thelight-transmitting cover is, however, dispersive, problems may bepresented. Therefore, the thickness t is preferably set to fall withinthe tolerance.

The coma (B) cannot be corrected by the objective lens. It is preferableto make the absolute value of the coma as small as possible. Even if thenumerical aperture NA increases, the thickness t of thelight-transmitting cover becomes small, so that the absolute value ofthe coma W₃₁ does not become large.

Each aberration of the objective lens scarcely poses any problem evenwhen the numerical aperture NA is increased. According to the opticaldisc system described above, recording and/or reproduction can thus beperformed in higher density than those or that of a conventional system.

According to another aspect of the present invention, there is provideda magneto-optical disc system for recording an information signal in amagneto-optical disc and/or reproducing an information signal from themagneto-optical disc, the magneto-optical disc having a substrate, amagneto-optical recording layer formed on the substrate and irradiatedwith a laser beam for recording or reproduction, and alight-transmitting cover which is formed to cover the magneto-opticalrecording layer and through which the laser beam is transmitted, and themagneto-optical recording system comprising laser beam generating meansfor generating the laser beam, an objective lens for focusing the laserbeam on the magneto-optical recording layer through thelight-transmitting cover, and magnetic field applying means for applyinga magnetic field to the magneto-optical layer, characterized in that thelight-transmitting cover of the magneto-optical disc has a thicknessfalling within a range of 0.6 to 0.1 mm, and a numerical aperture of theobjective lens falls within a range of 0.55 to 0.70.

In the system using the optical disc and the magneto-optical disc ofthis invention, the numerical aperture NA of the objective lens forperforming high-density recording or reproduction falls within the rangeof 0.55 to 0.70, so that the thickness of the light-transmitting coverof the disk, formed to cover the recording layer and transmit the laserbeam to the recording layer, is set to fall within the range of 0.6 to0.1 mm accordingly.

Thus, the optical and magneto-optical recording media capable ofperforming high-density recording or reproduction can be realized.

In the magneto-optical disc, if a high-permeability layer is formedbetween the substrate and the magneto-optical layer, vertical magneticfield efficiency is preferably increased.

The magneto-optical disc may have respective recording layers on bothsides of the substrate. Such magneto-optical disc as to have therespective recording layers on both the sides can be used in amagneto-optical disc system of this invention. In this system, a pair ofoptical pickup units, each of which have a laser beam generating means,an objective lens, and a magnetic field applying means, are arranged onopposite sides of a magneto-optical disc so as to face each other.

According to the magneto-optical disc having the recording layers on itsboth surfaces, and the magneto-optical disc system capable of using thismagneto-optical disc, higher-capacity recording and reproduction can beperformed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing a basic arrangement of a conventionalmagneto-optical disc system;

FIG. 2 is a sectional view showing an arrangement of a magneto-opticaldisc system according to a first embodiment of the present invention;

FIG. 3 is a sectional view showing an arrangement of a magneto-opticaldisc system according to a second embodiment of the present invention;

FIG. 4 is a front view of an optical glass having a coil pattern used inthe magneto-optical disc systems of FIGS. 2 and 3 and a magneto-opticaldisc system of FIG. 5; and

FIG. 5 is a sectional view showing the magneto-optical disc systemaccording to a third embodiment of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

The first to third embodiments of this invention will be described withreference to FIGS. 2 to 5.

FIG. 2 is a sectional view showing the first embodiment in which thepresent invention is applied to a magneto-optical disc system. FIG. 2shows the basic arrangement of this magneto-optical disc system.

As shown in FIG. 2, this magneto-optical disc system comprises anobjective lens 2 having an NA of 0.55 to 0.70 and a magnetic fieldgenerating unit 9 constituted by a light-transmitting optical glass 8having a coil pattern 7.

This magneto-optical disc system is arranged to perform recording andreproduction upon radiating a laser beam of the laser beam unit to adisc-like one-sided magneto-optical recording medium 40, which isobtained by sequentially stacking a dielectric layer 42, a magneticrecording layer 43, a dielectric layer 44, a reflecting film 45, and aprotective cover 46 on a thin light-transmitting substrate 41 having athickness t₂ of 0.1 to 0.6 mm which is smaller than that of theconventional substrate. The magnetic field generating unit 9 employs amagnetic field modulation scheme and applies a magnetic field to themagnetic recording layer 43 (to be described later in detail withreference to FIG. 4).

The thickness of the dielectric layer 42 is much smaller than thethickness t₂, so that it will be neglected with respect to the thicknesst₂.

The objective lens 2 serving as an optical system and the optical glass8 serving as a magnetic system are bonded and fixed to a bobbin (notshown) to be integrated. The coil pattern 7 is arranged close to themagneto-optical recording medium 40.

Since the thickness t₂ of the light-transmitting substrate 41 is muchsmaller than the thickness of the conventional substrate, and the coilpattern 7 is located near the light-transmitting substrate 41, adistance between the coil pattern 7 and the magnetic recording layer 43is preferably made small. Since the optical system can be formedintegrally with the magnetic system, a magneto-optical pickup unitconstituted by these optical and magnetic systems can be made compactand not expensive. Recording in and reproduction from double-sidedmagneto-optical recording media can be performed as will be described inthe second and third embodiments.

In a magneto-optical recording medium used in this magneto-optical discsystem, for example, a 3.5" magneto-optical disc is preferably used as ahard disc or is preferably housed in a cartridge holder.

When the NA of the objective lens 2 is increased to fall within therange of 0.55 to 0.70, its focal depth (=λ/NA², λ: wavelength of laserbeam) becomes small. Since the magneto-optical disc is made small asdescribed above, an actuator (not shown) constituting a drive system forthe magneto-optical pickup can be made small, and its frequencycharacteristics can be improved. As this actuator can properly performtracking control of the magneto-optical pickup, it raises no problemthat the focal depth is small.

The spherical aberration W₄₀ is corrected by the objective lens 2. Asfor the coma W₃₁, even if the NA of the objective lens is increased asdescribed above, it raises no problem, because the thickness t₂ of thelight-transmitting substrate is quite small.

Numerical apertures NA and thicknesses t at the time of the coma W₃₁being equivalent to the coma W₃₁ at the time when NA of the objectivelens is 0.5 and the thickness t of the light-transmitting substrate(light-transmitting cover) is 1.2 mm are obtained in the following fourcases.

                  TABLE 1                                                         ______________________________________                                        NA      0.50      0.55   0.6     0.65 0.70                                    t (mm)  1.2       0.9    0.69    0.55 0.44                                    ______________________________________                                    

As shown in Table 1, if the thickness t is set to fall within the rangeof 0.6 to 0.1 mm even when the NA range is 0.55 to 0.70, the coma can besuppressed to a level equal to or lower than that of the conventionalcase, so that no problem raises.

As the NA of the objective lens 2 is increased, the thickness of theobjective lens 2 is increased. However, since the thickness t₂ of thetransparent substrate is small, a distance d (working distance) betweenthe objective lens 2 and the magneto-optical recording medium 40 asshown in FIG. 2 increases more than a predetermined value. The objectivelens 2 i not brought into contact with the magneto-optical recordingmedium 40, and the optical glass 8 can be interposed between theobjective lens 2 and the magneto-optical recording medium 40.

Dust is a perplexing problem to an optical disc system can be eliminatedwhen a cartridge or hard disc medium is employed as the magneto-opticalrecording medium.

The grain size and distribution of the dust are important factors toconsider in the performance of the optical disc system. A radius r (FIG.2) of a circle formed when a laser beam is projected on the transparentsubstrate 41 is given as follows:

    r=t·tan (arc sin (NA/N))                          (4)

As is apparent from equation (4), when the thickness t of thelight-transmitting cover is decreased, the radius r is also reduced. Butthe NA is large, so that the radius r cannot be greatly decreased.Therefore, no dust problem is posed in this embodiment.

As described above, in the magneto-optical disc system of thisembodiment, the NA of the objective lens is set to fall within the rangeof 0.55 to 0.70 and, at the same time, the thickness t₂ of thelight-transmitting substrate (light-transmitting cover) is set to fallwithin the range of 0.6 to 0.1 mm, so that the recording density can beincreased (0.55/0.50)² to (0.70/0.50)² times, nearly 1.2 to 2 timeslarger than that of the conventional system, in which an objective lenshaving an NA of 0.50 is used, as is apparent from equation (1). All theabove-mentioned various problems caused by an increase of NA can thus beapparently solved. Therefore, a large storage capacity magneto-opticaldisc system can be obtained without posing any problem.

Detailed arrangements of the magneto-optical disc system given in thefirst embodiment will be further given in a second and thirdembodiments.

FIG. 3 is a sectional view of a magneto-optical disc system according tothe second embodiment of the present invention.

In the second embodiment, a first and a second magneto-optical pickupunits, each of which has an optical system and a magnetic system, arearranged on opposite sides of a magneto-optical recording medium 10 soas to face each other as shown in FIG. 3.

The first magneto-optical pickup unit located above the magneto-opticalrecording medium 10 and the second magneto-optical pickup unit locatedbelow the magneto-optical recording medium 10 are substantially the sameas described below, and thus, reference numerals 1 to 9 correspond toreference numerals 1' to 9', respectively.

Each of the first and second magneto-optical pickup units of FIG. 3comprises a pickup that is an optical system and has a laser unit 1, 1',an objective lens 2, 2' having an NA of 0.55 to 0.70, a bobbin 4, 4' onwhich a focus coil 3a, 3a' and a tracking coil 3b, 3b' are wound, and amagnet 5, 5' which surrounds the bobbin 4, 4', and magnetic fieldgenerating units 9, 9'.

The bobbin 4, 4' of the pickup 6, 6' is of cylindrical shape, and on thebobbin 4, 4' are wound the focus coil 3a, 3a' for driving the pickup 6,6' in a direction of double-headed arrow F in FIG. 3, and the trackingcoil 3b, 3b' for driving the pickup 6, 6' in a direction of adouble-headed arrow T in FIG. 3. Lens support members 2a and 2a' arearranged adjacent to end portions 4a and 4b' of the coil bobbins 4 and4', respectively. The objective lenses 2 and 2' for focusing the laserbeams emitted from the laser units 1 and 1' are supported by the lenssupport members 2a and 2a', respectively.

As shown in FIG. 4, the magnetic field generating units 9 and 9' areformed such that conductors 7a and 7a' for generating magnetic fieldsupon reception of currents of high-frequency signals are formed asspiral coil patterns 7 and 7' in upper surfaces 8a and 8a' oflight-transmitting optical glasses 8 and 8' made of, e.g., quartz,respectively. The objective lenses 2 and 2' are positioned on lowersurfaces 8b and 8b' having no coil patterns 7 and 7' of the opticalglasses 8 and 8' so that the centers of the objective lenses 2 and 2'are aligned with the centers of the coil patterns 7 and 7',respectively. The objective lenses 2 and 2' are fixed near the endportions 4a and 4a' of the coil bobbins 4 and 4' through the lenssupport members 2a and 2a', respectively. The optical glasses 8 and 8'are adhered to fix to the end portions 4a and 4a' of the coil bobbins 4and 4', respectively. Therefore, the laser beams can be focused on thecenters of the generated magnetic fields, respectively. Thus, anycentering process is not needed during assembly of the pickups.

The Quartz was used as a material for the optical glasses 8 and 8', butquartz can be replaced with other materials if they havelight-transmitting properties. As the coil patterns 7 and 7', printedcoils, thin film coils, or the like may be used for example. Holes fortransmitting laser beams from the laser units 1 and 1' may be formed inthe optical glasses 8 and 8' so as to prevent the laser beams from beingreflected by the surfaces of the optical glasses 8 and 8', respectively.

The first two-sided magneto-optical recording medium 10 shown in FIG. 3is the one proposed in the specification and drawings of Japanese PatentApplication No. 1-142563 filed by the present applicant. Recordingportions 16, photo-curable resin layers 17, and transparent protectiveplates 18 are formed respectively on both surfaces of one substrate 11.Both the transparent protective plates 18 and the photo-curable resinlayers 17 have light-transmitting properties and a total thicknessthereof can be less than 0.6 mm.

In each of the recording portions 16, the reflecting layer 15 isprovided closer to the substrate 11 than a magnetic recording layer 12,and further, the reflecting layer 15, a second dielectric layer 14, therecording layer 12, and a first dielectric layer 13 are stacked in turnfrom the side of the substrate 11.

In this double-sided magneto-optical recording medium 10, the substrate11 is used in common, so that the thickness of the recording medium canbe made substantially half as compared with that of a conventionaldouble-sided magneto-optical recording medium, in which the twosubstrates are stuck together.

The function of the magneto-optical disc system will be described below.

In the first magneto-optical pickup unit, as a current is supplied tothe focus coil 3a, the coil bobbin 4 and the magnetic field generatingunit 9 integrated with the coil bobbin 4 are driven to displace alongthe optical axis of the objective lens 2, that is, in a direction ofarrow F in FIG. 3. When a current is supplied to the tracking coil 3b,the coil bobbin 4 and the magnetic field generating unit 9 are driven todisplace in a direction perpendicular to the optical axis of theobjective lens 2, which is indicated by arrow T in FIG. 3.

Similarly, the second magneto-optical pickup unit is driven to displacein the directions of arrows F and T synchronously with the displacementof the first magneto-optical pickup unit.

At the same time high-frequency current of signals obtained byamplifying signals to be recorded is supplied to the coil patterns 7 and7' of the magnetic field generating units 9 and 9', which are made ofthe conductors 7a and 7a', so that magnetic fields are generated,respectively. These magnetic fields are obtained by high-speed reversecontrols exercised in accordance with the recording signals, and areapplied to the magnetic recording layers 12 and 12' of the double-sidedmagneto-optical recording medium 10. The laser beams emitted from thelaser units 1 and 1' are focused on areas of the magnetic recordinglayers 12 and 12', where the magnetic fields are applied to, through theobjective lenses 2 and 2' and the light-transmitting optical glasses 8and 8'. The temperatures of the magnetic recording layers 12 and 12' areraised over the Curie point to perform recording the informationsignals.

As is apparent from the above description, the magneto-optical discsystem given in the embodiment is arranged such that the optical systemcomprising the pickup 6, 6' which has the laser unit 1, 1'; theobjective lens 2, 2' and so forth, and the magnetic system comprisingthe magnetic field generating unit 9, 9', which is formed by providingthe coil pattern 7, 7' in the optical glass 8, 8', are arranged on thesame side with respect to the double-sided magneto-optical recordingmedium 10. Thus, the distance between the coil pattern 7, 7' and themagneto-optical recording medium 10 can be shortened. Since thethickness of the light-transmitting cover comprising the transparentprotective plate 18 and the photo-curable resin layer 17 is small, thedistances between the coil pattern 7, 7' and the magneto-opticalrecording layer 12 of the magneto-optical recording medium 10 can bereduced, so that recording in the double-sided magneto-optical recordingmedium by means of the magneto-optical modulation scheme can beperformed, though in the conventional case, it has been difficult sofar.

If the coil bobbin 4, 4' of the pickup 6, 6', and the magnetic fieldgenerating unit 9, 9', are respectively stuck together, for example, byan adhesive or the like, the center of the objective lens 2, 2' iseasily aligned with the center of the magnetic field of the magneticfield generating unit 9, 9'. Since the pickup 6, 6' is interlocked withthe magnetic field generating unit 9, 9' by a focus servo, the intensityof the magnetic field applied to the double-sided magneto-opticalrecording medium 10 can always be made constant. A drive system fordriving the magnetic system, which is required in a conventional case,can be omitted. Since the space of the magneto-optical pickup unit canbe saved, freedom in design can be increased.

According to the second embodiment, recording in or reproduction fromthe double-sided magneto-optical recording medium 10 can be variouslyperformed. For example, if the laser units 1 and 1' and the magneticfield generating units 9 and 9' are simultaneously used, simultaneousrecordings in the respective upper and lower surfaces of thedouble-sided magneto-optical recording medium 10 can be performed. Whenboth the laser units 1 and 1' are simultaneously used, simultaneousreproduction can be performed. A large storage capacity recording mediumcan be obtained, and high-speed recording and reproduction ofinformation signals can be performed.

After recording in or reproduction from one surface of the double-sidedmagneto-optical recording medium 10 is performed, recording in orreproduction from the other surface can be performed. The capacity ofrecording and reproduction can be doubled as compared with that of aone-sided magneto-optical recording medium.

In the above case, when recording in one surface is being performed, itis possible to put both the magnetic field generating units 9 and 9' inaction, and thereby, apply the two magnetic fields to one of themagnetic recording layers 12 from above and below the layer. Thus, therecording can be made in a stronger magnetic field.

Recording can be performed by using only one of the magnetic fieldgenerating units located near one side of the medium when the recordingis being performed on the other side thereof.

The third embodiment will be described below. In this embodiment, thesame magneto-optical disc system as illustrated in FIG. 3 is used asshown in FIG. 5, but recording and reproduction can be performed on asecond double-sided magneto-optical recording medium 30 shown in FIG. 5.

The second double-sided magneto-optical recording medium 30 wasdisclosed in the specification and drawings of Japanese PatentApplication No. 1-274734 filed by one of the inventors of thisapplication and other persons. In the recording medium 30, ahigh-permeability layer 32, a photo-curable resin layer 33, amagneto-optical recording layer 34, an adhesive layer 35, and atransparent protective plate 36 are sequentially stacked on each of thesurfaces of a single substrate 31. Both the transparent protective plate36 and the adhesive layer 35 have light-transmitting properties, and atotal thickness thereof is 0.6 mm or less.

The high-permeability layer 32 is made of a transition metal such as Fe,Co, Ni or the like, or an alloy of the transition metals, such asPermalloy, Sendust, or an amorphous magnetic alloy. Vertical magneticfield efficiency along the vertical direction of the double-sidedmagneto-optical recording medium 30 can be increased.

In the third embodiment, the second double-sided magneto-opticalrecording medium 30 has the high-permeability layer 32, so that magneticflux from the magnetic field generating units 9 and 9' forms a closedmagnetic loop as indicated by a broken line in FIG. 5 for example. Inthe recording mode, the magnetic flux applied to the double-sidedmagneto-optical recording medium 30 can thus be effectively bundled andpreferably increase the vertical magnetic field efficiency.

Also in the third embodiment, recording in or reproduction from bothsurfaces of the double-sided magneto-optical recording medium 30 can besimultaneously performed. Further, as recording in or reproduction fromone surface is performed, recording in or reproduction from the othersurface can be performed.

As described above, in the second and third embodiments, the numericalaperture NA of the objective lens 2, 2' is increased as embodied in thefirst embodiment, so that high-density recording and reproduction arepossible. At the same time, the double-sided magneto-optical recordingmedium can be used, so that high storage capacity, higher-speedrecording and reproduction are possible. Thus, a large storage capacitymagneto-optical disc system can be proposed.

In the first to third embodiments, the magnetic field generating unit 9,9' utilized the magnetic field modulation scheme, but another schemesuch as an optical modulation scheme can be utilized by the magneticfield generating unit 9, 9'.

The magneto-optical disc system is embodied in the first to thirdembodiments, but the present invention is not limited to such system,and applicable to an optical disc system, in which recording and/orreproduction are performed on an optical disc, an optical recordingmedium (including a "write-once" type optical recording medium forexample) that has pits in a recording layer.

According to the optical disc system of the present invention, the NA ofthe objective lens for focusing the laser beam is increased, and thethickness of the light-transmitting cover of the optical disc isdecreased, so that recording and/or reproduction can be performed inhigher density, and large-capacity recording can be realized.

We claim:
 1. A recordable optical disc, comprising:a recordable layeronto which a laser beam is converged by an objective lens having anumerical aperture (NA) falling within a range of about 0.55 to 0.70,the recordable layer having an optical characteristic that changes whenheated by the converged laser beam; and a light-transmitting substratesupporting the recordable layer, the light-transmitting substrate havinga thickness falling within a range of about 0.1 to 0.6 mm andtransmitting the laser beam therethrough to the recordable layer.
 2. Arecordable optical disc according to claim 1, wherein the recordablelayer is comprised of a material that forms a pit after heating by theconverged laser beam.
 3. A recordable optical disc according to claim 1wherein the recordable layer is supported by the light-transmittingsubstrate such that a peripheral edge of the recordable layer islaterally spaced apart from a peripheral edge of the light-transmittingsubstrate to form a spaced portion therebetween.
 4. A recordable opticaldisc according to claim 3, further comprising an adhesion layer providedon the light-transmitting substrate between the light-transmittingsubstrate and the recordable layer, the adhesion layer occupying thespaced portion surrounding the peripheral edge of the recordable layer.5. A recordable optical disc according to claim 1, further comprising:asecond recordable layer onto which a second laser beam is converged by asecond objective lens having a numerical aperture (NA) falling within arange of about 0.55 to 0.70, the second recordable layer having anoptical characteristic that changes when heated by the second convergedlaser beam; and a second light-transmitting substrate supporting thesecond recordable layer, the second light-transmitting substrate havinga thickness falling within a range of about 0.1 to 0.6 mm andtransmitting the second laser beam therethrough to the second recordablelayer.
 6. A recordable optical disc according to claim 5, furthercomprising interconnecting means for interconnecting the twolight-transmitting substrates.
 7. A recordable optical disc according toclaim 5, wherein the second recordable layer is comprised of a materialthat forms a pit after heating by the second converged laser beam.
 8. Arecordable optical disc according to claim 5, wherein the secondrecordable layer is supported by the second light-transmitting substratesuch that a peripheral edge of the second recordable layer is laterallyspaced apart from a peripheral edge of the second light-transmittingsubstrate to form a second spaced portion therebetween.
 9. A recordableoptical disc according to claim 8, further comprising a second adhesionlayer provided on the second light-transmitting substrate between thesecond light-transmitting substrate and the second recordable layer, thesecond adhesion layer occupying the second spaced portion surroundingthe peripheral edge of the second recordable layer.